That said, compiler errors are only part of the benefit. The other huge area where this would help is around tooling/IDE support. Sencha's done some cool things with their Eclipse plugin, no doubt. But anyone who's worked with Java or Groovy using IDEA or Eclipse, or C# using Visual Studio, knows that Sencha's plugin is barely scratching the surface of what is possible. And all of that power is predicated upon some kind of type inference.
It also doesn't help that for reasons I'm still unable to fathom, Sencha is limiting the ability to buy their Eclipse plugin. But that's a completely different topic/rant.